1869.] 



of the Solar Prominences. 



65 



sights themselves are so beautiful and interesting that no other incentive is 



needed. This morning I showed a magnificent prominence upwards of 



3' in height, and she testifies that my sketches do not do them justice. 



The instrument I am now using is the Royal Society's spectroscope as 

 fitted up for the eclipse ; but I have increased the dispersion nearly three- 

 fold by inserting four compound prisms (extracted from the hand-spectro- 

 scopes). These amount to 7 inches of glass and sixteen surfaces ; so you 

 may imagine that there is some loss of light and definition. I have also 

 had to shorten the focal distance (and therefore diminish the magnifying- 

 power) by interposing a hand-telescope's object-glass — an additional ob- 

 struction and complication. I lost a great deal of fine weather (of which I 

 get very little now) while trying to perfect this arrangement. I can still 

 further increase the dispersion (without much loss of definition for mono- 

 chromatic light) by turning the main prism, and so departing from the po- 

 sition of minimum deviation. But this is a resource which I keep to go 

 on with when I tire of the advantage I have gained already. 



The long train of compound prisms (as at present arranged) unfortunately 

 bars me from the violet end of the spectrum. This is unfortunate, as it 

 would be in the highest degree interesting to compare the a and y images . 

 Some day I shall get impatient, pull the whole affair to pieces, and arrange 

 afresh with this object. As it is, I have to be very chary of quitting 

 beaten ground, as we boast of no instrument-makers here ! 



I wish I had time to write fully and connectedly on the subject. It is 

 only necessary to put people on the track. It is one easily followed, and 

 will amply repay any expenditure in arranging prisms to get a maximum 

 dispersion, for there is any amount of light. 



VOL. XVIII. 



F 



